1. Field of the Invention
The present invention relates to a PWM controller for an inverter, which forms PWM control signals for controlling a PWM inverter from PWM pattern data read out of a memory.
2. Description of the Prior Art
In a conventional controller for a pulse width modulation (PWM) inverter, the desired control pattern data is calculated and is stored in a memory in advance in off-line, and the control pattern data is read out of the memory according to an instruction in on-line to control the PWM inverter, as disclosed in the report "A Semiconductor Power Inversion Circuit" edited by the Professional Committee for the Semiconductor Power Inversion System Research in the Electric Society, Mar. 31, 1987. In this system, since the desired control pattern data is calculated in off-line, the controls such as the low-dimensional harmonic elimination PWM control, the torque ripple reduction control, the harmonic loss reduction control and the like, while it is difficult to perform such controls in another PWM control in on-line, can be readily carried out.
However, in the conventional PWM controller having a memory therein, only a synchronous type in which the PWM frequency is proportional to the inverter frequency, is used. When the operating frequency becomes low, the torque ripple increases, and there is a limitation in the minimum frequency. That is, the memory is addressed according to a voltage command V* and an electrical angle command .theta.*, and the PWM control signals corresponding to these two commands V* and .theta.* are output. When the command .theta.* is given by n bits, the 2.sup.n number of commands .theta.*(=.theta.0*, .theta..sub.1 *, .theta..sub.2 *, . . . , .theta..sub.k *, . . . , .theta..sub.2 n.sub.-2 *, .theta..sub.2 n.sub.-1 *) are available. When the frequency is lowered, the staying time of the signal in .theta..sub.k * (k.apprxeq.0.sub.1 to 2.sup.n -1) becomes long, that is, since the phase resolution is constant irrespective of the operating frequency, as the operating frequency is lowered, the resolution per one unit time reduced. Hence, the current and the torque ripple increase, and it becomes impossible to conduct the operation due to the overcurrent below a certain frequency determined by the capacity of the memory, which restricts its applications.
It is desirable that the operating frequency is wide in an inverter widely used in engineering machines, and the asynchronous PWM control should be used in the low speed range. However, providing the two controller, i.e., the PWM controller using the memory and the asynchronous PWM controller brings about the cost increase.